The problem is the Y pipe, period. There is no loss in power going to larger and larger pipe after the merfer. The merger must expand too fast, causing the exhaust gases to cool and slow down, which is why there is a loss of low end power. I've pointed this out before.
The Single 3" pipe has just a bit more surface area than dual 2" pipes, and when the merger is designed correctly, there should be no loss in power. Period.
From Bimmerworlds graph, I don't see a loss in power, but I do see a dyno run that was started later than the inital baseline, which causes a huge dip at the begining as the engine revs under load.